The Chandra site has a field guide to supernovas:http://chandra.harvard.edu/xray_sources/supernovas.html
which explains the process by which the star evolves and explodes.
Another site you may appreciate is at the University of Michigan, again
on the evolution of stars:http://www.umich.edu/~gs265/star.htm
If you consider that even the most massive stars, with the shortest
lifetimes before exploding, still live for about 2 million years, then
you can see that it would be an amazingly lucky coincidence if we
happened to look with a telescope at a star at the exact moment it
began to show signs of pre-explosion activity. Although, as you'll
read in the Chandra site mentioned above, if we had had a telescope in
the year 1054, we would have observed an event just like this. We may
get lucky again in our lifetime!